how cells reproduce
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Chapter 11. How Cells Reproduce. S phase (DNA synthesis; chromosome duplication). Interphase: metabolism and growth (90% of time). G 1. G 2. Mitotic (M) phase: cell division (10% of time). Cytokinesis (division of cytoplasm). Mitosis (division of nucleus). Figure 8.6. - PowerPoint PPT PresentationTRANSCRIPT
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BiologyConcepts and Applications | 9eStarr | Evers | Starr
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Chapter 11Chapter 11
How Cells ReproduceHow Cells Reproduce
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Cytokinesis(division ofcytoplasm)
Mitosis(division of nucleus)
Mitotic (M) phase:cell division(10% of time)
Interphase: metabolism andgrowth (90% of time)
S phase (DNA synthesis;chromosome duplication)
G1 G2
Figure 8.6
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11.1 How Do Cells Reproduce?
• Multiplication by division– A life cycle is a collective series of events that
an organism passes through during its lifetime
– Even single cells of multicellular organisms have a life cycle
– Cell cycle: a series of events from the time a cell forms until its cytoplasm divides
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Multiplication by Division (cont’d.)
• A typical cell spends most of its life in interphase: interval between mitotic divisions when a cell grows– During interphase, a cell roughly doubles the
number of its cytoplasmic components, and replicates its DNA
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Multiplication by Division
• Mitosis: process of nuclear division that maintains chromosome number
• Mitosis and cytoplasmic division are the basis of:– Developmental processes (e.g., increases in
body size and tissue remodeling)
– Replacement of damaged or dead cells
– Eukaryotic asexual reproduction (offspring are produced by one parent)
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Multiplication by Division
• When a cell divides by mitosis, it produces two descendant cells– Each with the same number and type of
chromosomes as the parent
• Human body cells are diploid (contain pairs of chromosomes)– With exception, the chromosomes of each
pair are homologous: have the same length, shape, and genes
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Multiplication by Division
• During mitosis, the sister chromatids are pulled apart– Each sister chromatids end up in separate
nuclei that are packaged into separate cells
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Multiplication by Division
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A Pair of homologouschromosomes in a cellduring G1. Both areunduplicated.
B By G2, eachchromosome hasbeen duplicated.
C Mitosis and cyto-plasmic division package one copy of each chromosome intoeach of two new cells.
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Control Over the Cell Cycle
• Whether or not a cell divides is determined by mechanisms of gene expression control– “Brakes” on the cell cycle normally keep the
vast majority of cells in G1
– “Checkpoint genes” monitor:• The completion of DNA copying
• DNA damage
• Nutrient availability
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11.2 What Is the Sequence of Events During Mitosis?
• Before mitosis:– Interphase: chromosomes are loosened to
allow transcription and DNA replication
– Early prophase: in preparation for nuclear division the chromosomes begin to pack tightly
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What Is the Sequence of Events During Mitosis?
• Prophose (first stage of mitosis):– Chromosomes further condense
– One of the two centrosomes move to the opposite end of cell
– Microtubules assemble and lengthen, forming a spindle (functions to move chromosomes)
– Nuclear envelope breaks up
– Sister chromatids are attached to opposite centrosomes
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What Is the Sequence of Events During Mitosis?
• Final stages of mitosis:– Metaphase: all chromosomes are aligned
midway between spindle poles
– Anaphase: sister chromatids separate and move toward opposite spindle poles
– Telophase: chromosomes arrive at opposite spindle poles and decondense; two new nuclei form
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11.3 How Does a Eukaryotic Cell Divide?
• In most eukaryotes, cytokinesis (cytoplasmic division) occurs between late anaphase and the end of telophase
• The process of cytokinesis differs between plant and animal cells
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How Does a Eukaryotic Cell Divide?
• Animal cell cytokinesis:– Typical animal cells pinch themselves in two
after nuclear division ends• The spindle begins to disassemble during
telophase
• Contractile rings drag the plasma membrane inward
• Cleavage furrow (indentation) forms
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How Does a Eukaryotic Cell Divide?
• Plant cell cytokinesis:– Dividing plant cells face a challenge because
a cell wall surrounds their plasma membrane
– By the end of anaphase short microtubules form on either side of the future plane of division
– Disk-shaped structure called the cell plate forms and eventually partitions the cytoplasm
– The cell plate forms into two new cell walls
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11.4 What Is the Function of Telomeres?
• 1997: geneticist Ian Wilmut and his team cloned the first mammal, a lamb named Dolly, from an adult somatic cell
• Although Dolly was healthy at first, she showed signs of premature aging– Arthritis, lung disease, etc.
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What Is the Function of Telomeres? (cont’d.)
• Dolly’s early demise may have been the result of abnormally short telomeres– Telomeres are noncoding repeat DNA
sequences (repeated thousands of times) found at the ends of eukaryotic chromosomes
– Telomere sequences provide a buffer against the loss of more valuable internal DNA
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Cleavagefurrow
SE
M
Cleavage furrowContracting ring ofmicrofilaments
Daughter cellsFigure 8.8a
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Daughter cells
New cell wallVesicles containingcell wall material Cell plateCell wall
Wall ofparent cell
Cell plateforming
Daughternucleus
LM
Figure 8.8b
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What Is the Function of Telomeres?
• A telomere buffer is very important– Typically, a eukaryotic chromosome shortens
by about 100 nucleotides with each DNA replication
– When chromosomes contain telomeres that are too short, checkpoint gene products halt the cell cycle and cell death soon follows
• Most body cells can divide only a certain number of times before this happens
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What Is the Function of Telomeres?
• Telomeres-dependent cell division limits may protect against uncontrolled cell division– Keeps “uncontrolled” cells from overrunning
the body
• Cell division limits vary by species and may set an organism’s life span
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What Is the Function of Telomeres?
• A few normal adult cells retain the ability to divide indefinitely, replacing cell lineages that die out– These immortal cells are called stem cells
• Stem cells continuously produce enzymes called telomerases– Telomerases reverse the telomere shortening
that normally occurs after DNA replication
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What Is the Function of Telomeres?
• Mice genetically engineered to have no telomerase enzymes age prematurely– Life expectancy declines by about half
• Introducing telomerase back in these mice rescues their vitality– Decrepit tissue repairs itself and functions
normally; reproduction becomes possible
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What Is the Function of Telomeres?
• Although telomerase holds therapeutic promise for the rejuvenation of aged tissues, it can also be dangerous– Cancer cells characteristically express high
levels of telomerase
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The Role of Mutations
• When enough checkpoint mechanisms fail, a cell loses control over its cell cycle– Interphase may be skipped, so division occurs
over and over with no resting period
– Signaling mechanisms that cause abnormal cells to die may stop working
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The Role of Mutations
• Neoplasm: accumulation of abnormally dividing cells
• Tumor: neoplasm that forms a lump
• Oncogene: gene that helps transform a normal cell into a tumor cell
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Cancer
• Benign neoplasms such as warts are not usually dangerous– Slow growth
– Cells retain plasma membrane adhesion proteins
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Cancer
• A malignant neoplasm get progressively worse and is dangerous to health
• Characteristics of malignant cells:– Abnormal growth and development
– Altered cytoplasm and plasma membrane
– Cells undergo metastasis: process in which malignant cells spread from one part of the body to another
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Cancer
• Cancer: disease that occurs when a malignant neoplasm physically and metabolically disrupts body tissues– Each year, cancer causes 15 to 20 percent of
all human deaths in developed countries
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Cancer
• Lifestyle choices can reduce one’s risk of acquiring mutations:– Do not smoke
– Avoid sun exposure
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Cancer
• Some neoplasms can be detected with periodic screening such as gynecology or dermatology exams– If detected early enough, many types of
malignant neoplasms can be removed before metastasis occurs
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Cancer
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A Basal cell carcinomais the most common type of skin cancer. This slow-growing, raised lump may be uncolored, reddishbrown, or black.
B Squamous cell carci-noma is the second mostcommon form of skincancer. This pink growth,firm to the touch, growsunder the surface of skin.
C Melanoma spreadsfastest. Cells form dark,encrusted lumps thatmay itch or bleed easily.
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11.6 Application: Henrietta’s Immortal Cells
• Finding human cells that grow in a laboratory took George and Margaret Gey nearly 30 years to find
• In 1951, an assistant prepared a cell line that divided vigorously– These cells were named HeLa cells, after the
patient from whom the cells had been taken: Henrietta Lacks
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Application: Henrietta’s Immortal Cells
• Although Henrietta passed away from cervical cancer, her cells lived on in the Geys’ laboratory, aiding in polio studies
• HeLa cells are still used today in laboratories all over the world to study cancer and numerous other cellular processes– HeLa cells contributed to the work of Nobel
Prize winners
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